How can I use Webmail on 80 port

I have install ispconfig on an old server in the company. It work well and smooth. However, the webmail use 81 port as the admin panel. Can I use another port for webmail, e.g. 80 port. As for security issue, my company inform me not to use 81 again.

I have read the article about apache proxy before, but it not work on my system. I am using Mandriva 2006 free. However, it not matter.I don't need to change the admin panel to 80 port. I just want to use webmail on 80 only.